The Combat Airship is an ultimate introduced in the 13.0.0 update.
It appears to be a large, wooden airship with some yellow, beige and orange details. It has a large air balloon on the top of it, and it is colored white.
It is above the fort and attacks from the guns and mortars the area in front of the fort.
At this moment, ultimates like this are not in the game. It is expected that ultimates will be added to the game at some point due to the fact that the player is able to browse them in the clan fort section, and also in the clan storage, which is quite misleading. At the moment, it is unsure if an ultimate will be an ability used once per game, or if it will be constant.